I have a Huawei E1752 HSDPA broadband modem USB stick that works great if plugged into my laptop and connected via the ISP vendor's proprietry software. If I plug the stick into my 3G Router (TP-Link TL-MR3220) the connection sometimes works, but oftend "fails". When it "fails" I get assigned an IP address (outside my local netmask) and some reasonable looking DNS servers, but I can't ping any outside addresses or access the internet. Sometimes when connecting through the router I get assigned 10.11.12.13 as a DNS which obviously won't get me anywhere.
Where can I start to debug this problem?

4 Answers
4

One level is the TL-MR3220 which is assigning addresses to you locally. Quite likely it also NATs you to either the 3G or WAN connection.

The second level is your 3G provider which supplies an address etc to the TL-MR3220 3G side. Your 3G provider is probably NATing that access as well.

It is quite possible that 10.11.12.13, as odd as that might look, might be a reasonable DNS address supplied by your 3G provider and within your 3G provider's network.

When you have the 3G USB stick in your computer, look at what IP network you are on and what DNS you get, etc.

Then plug the 3G USB stick into the TL-MR3220 and see if, via what ever interface you get on the TL-MR3220, what ip network the 3G side of the TL-MR3220 gets, and, what DNS that side of the router gets, etc.

If there is a ping command on the TL-MR3220, can you ping somewhere outside?

Now, from your computer a ping of the TL-MR3220's wifi side should work. Your default route should be the address of the TL-MR3220's wifi side. The DNS could be the TL-MR3220's wifi side, or, it could be somewhere on the 3G provider's network, or it could be somewhere else.

Does a ping of an IP address outside your 3G provider's network work? Now can you do the same ping with a name rather than an address?

Thank you for your good advice for hunting down the problem. I tried most of the IP lookups and pingings as suggested before posting the question. There seemed to be no difference between the 3G side ip address and DNS addresses that gets assigned in both usage cases.
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JannieTJul 28 '11 at 15:36

It looks like the problem was with the ISP's DNS servers. Somehow, when I don't connect via their proprietry client interface, my request is blocked at the dynamically assigned DNS server. I changed the settings in my router to always use Google's DNS service (8.8.8.8 and 8.8.4.4) and haven't had the problem since.

I am using a TP-Link TL-MR3220 router with a Three Mobile UK 3G USB stick. It used to drop connection every 10 to 15 minutes. Now it has been working fine since I changed the Primary DNS to Google's 8.8.8.8 under the DHCP Settings as was suggested by JannieT.